The present invention relates to a scanning electron microscope, more particularly to a scanning electron microscope for obtaining high-resolution scanning images at a low acceleration voltage.
A scanning electron microscope (hereinafter shorted as SEM in this specification) obtains a magnified 2-dimensional scanning image of an object to be examined by emitting a beam of electrons from a heating type or a field emission type, concentrating it by an electrostatic or magnetic lens into a fine electron beam (a primary electron beam), applying the electron beam onto the object to be examined in a 2-dimensional scanning manner, detecting a secondary signal electrons that are secondarily generated or reflected from the object, and feeding the magnitude of the detected signal to the brightness modulation of a CRT tube which is scanned in synchronism with the primary electron beam.
A conventional SEM emits electrons from an electron source to which a negative voltage is applied, accelerates them by anodes of a grounding voltage, and applies the accelerated electrons (a primary electron beam) to an object to be examined at a grounding voltage.
Recently, semiconductor chips have become smaller and their circuit patterns have become extremely fine. Accordingly, SEMs have been widely used in place of optical microscopes to inspect manufacturing processes of semiconductor chips and processed semiconductor chips (e.g. by measurement of dimensions by electron beams and inspection of electric operations),
Semiconductor specimens to be examined by the SEM are generally made of a multiple layers of electrically-insulating materials on a conductor such as aluminum or silicon. When an electron beam is applied to such a specimen, the surface of the specimen is charged up, which will change the direction of motion of the emitted secondary electrons or the primary electrons themselves. Consequently, the resulting images may have an extraordinary contrast, distortion, or the like. To reduce the influence by this charging up, the energy of the emitted electron beam must be made as low as possible.
However, if the energy of the emitted electron beam (an acceleration voltage) is made low, a chromatic aberration due to energy dispersion of the electron beam generates and the resolving power of the SEM drastically goes down, which makes observation at a high magnification harder.
As a means for solving such a problem, a technology for forming a deceleration field for electron beams (hereinafter called xe2x80x9cretardingxe2x80x9d in this specification) has been disclosed in Japanese Non-examined Patent Publication No. 06-139985 (1994).
xe2x80x9cRetardingxe2x80x9d is a technology of forming a deceleration field by increasing the voltage to accelerate the electron beam to the anodes and applying a negative potential to the object to be examined, finally setting the acceleration voltage to a comparatively low level, and thus preventing chromatic aberration and charging up.
Although this retarding technology can reduce the charging-up of a specimen and accomplish low chromatic aberration and high resolving power, it has the following demerits:
While the deceleration field which is formed by applying a negative voltage to the specimen can decelerate the motion of the primary electron beam, it accelerates the secondary electrons and the reflected electrons generated by the specimen. In other words, the secondary electrons as well as the reflected electrons are refracted to the electron beam with a high energy.
Japanese Non-examined Patent Publication No. 06-139985 (1994) has disclosed a technology as a means for detecting electrons having such a high energy. This technology places a micro channel plate (MCP) having an aperture to pass the primary electron beam with its detection surface opposite to the specimen. This technology also places an energy filter which selectively detects reflected electrons between the specimen and the detecting surface of the micro channel plate.
Some other technologies have also been disclosed. One of such technologies has been disclosed in Japanese Non-examined Patent Publication No. 09-171791 (1997). This technology causes high-energy electrons to collide against a reflecting plate, converts them into secondary electrons, then guides the secondary electrons into a detector. Japanese Non-examined Patent Publication No. 08-124513 (1996) has disclosed another technology which detects electrons by striking electrons directly against an electron multiplier tube.
However, as the scanning electron microscopes using these detection principles detect both secondary electrons and reflected electrons by an identical detector. Accordingly, this type of SEM cannot distinguish secondary electrons from reflected electrons clearly.
Although the technology disclosed in Japanese Non-examined Patent Publication No. 06-139985 (1994) can detect reflected electrons only or both secondary and reflected electrons by means of the energy filter, it can neither detect only secondary electrons which are accelerated by a deceleration field nor detect both secondary and reflected electrons independently and simultaneously without mixture of information specific to secondary and reflected electrons.
The secondary electrons and the reflected electrons respectively have specific information. Therefore, it has been desired to detect these electrons individually to get detailed information on the object to be examined.
A technology for detecting these electrons individually has been disclosed in Japanese Non-examined Patent Publication No. 07-192679 (1995). This technology deflects the direction of motion of the secondary electrons by a deflector, separates the secondary electrons from the reflected electrons, and detects the secondary electrons and the reflected electrons individually by corresponding detectors which are placed in the moving passages of the electrons.
However, the above-mentioned technology combined with the retarding technology will allow the secondary electrons to have as high energy as that of the reflected electrons and consequently, their moving tracks become almost the same and they cannot be separated from each other.
As explained above, it is apparent that any conventional technology disclosed in the above patent specifications is hard to detect the secondary electrons and the reflected electrons individually when combined with the retarding technology.
An object of the present invention is to provide a scanning electron microscope (SEM) employing a retarding technology which reduces charge-up of the specimen and accomplishes low chromatic aberration and high resolving power and capable of detecting secondary electrons independently of the other electrons.
The object of the present invention is accomplished by a scanning electron microscope consisting of an electron source, a lens for condensing a primary electron beam emitted from said electron source, detectors for detecting electrons which are generated by radiation of the primary electron beam condensed by said lens onto a specimen, a first decelerating means for decelerating the primary electron beam before the primary electron beam hits said specimen, a second decelerating means for decelerating the electrons generated by collision of electrons against the specimen, and deflectors for deflecting the electrons decelerated by said second decelerating means to said detectors.
The scanning electron microscope of the aforesaid configuration, even when it employs the retarding technology, can selectively detect electrons of lower energies among those generated by radiation of the primary electron beam (e.g. secondary electrons).